Doom source code
Before release, the source code was tidied up by Bernd Kreimeier; the source release includes a changelog of his contributions. Several documentation files are also included. The original plan for the source code release involved a book Kreimeier was to write on the Doom engine; however, due to Doom's decreasing relevance in the gaming community, it was eventually judged not to be a marketable idea, and the project was abandoned. The source code was then released to the public.
Because of legal issues regarding the DMX sound library developed by Paul Radek which was used for Doom, the release was of the source code to the Linux port of Doom. Despite this, within several months, several ports had been made back to DOS. As a result, many source ports exist.
 Source code structure
The source code can be divided into sections. Files and functions within the source code have one- or two-letter prefixes to denote their subsystem.
- Automap code
- Initialisation/general code
- "Finale" (end of game) and "screen melt" code.
- Main game loop/control
- Heads-up display
- System-specific code
- Miscellaneous (includes the menu)
- Game logic/behaviour
- Rendering engine
- Sound code
- Status bar
- General graphic rendering
- End-of level "intermission" screen
- WAD file loading
- Zone memory allocation system
The following are common prefixes for functions, although they do not denote a particular subsystem, and there are no files with these prefixes:
- Action functions invoked in sprite movement frames (these are the functions used in Dehacked "code pointers")
- Callback functions passed to P_BlockThingsIterator (see p_maputl.c)
- "Thinker" functions set to be called each clock tic for some purpose (eg, moving platforms or flickering lights)
A detailed list of files and their purposes can be found in Doom source code files.
 Doom Classic
Doom 3: BFG Edition incorporates the "Doom Classic" port allowing to play The Ultimate Doom and Doom II on 32- and 64-bit Windows, on the Xbox 360 and on the PlayStation 3. There are a few changes done to the source code and packaging here. Notably:
- Bugs introduced in the LinuxDoom code by Bernd Kreimeier's cleanup and changes are absent
- MUS2MIDI and TiMidity are incorporated for MIDI playback, instead of using DMX
- A few static limits have been raised, notably visplanes and drawsegs to accommodate for No Rest for the Living's levels
- Several #defines have been moved out of their original files and into either Precompiled.h or defs.h (both new files)
- The Master Levels and No Rest for the Living are handled as mission packs like TNT: Evilution and Plutonia
- The French language file is still present but emptied, French is removed from the language_t enum
- The code is available under the GPLv3 or later
Note: this should not be confused with the iOS port also named Doom Classic.